I am wondering if anyone knew any final (definitive) word on whether recording with the SD card slot (and an SD card) for 1080p All-I video on the 5d Mark iii actually works without any problems or loss of quality.

I remember there was a ton of discussion that the SD slot was much slower than the CF slot (like 30mbps or something around there) and there was also talk that All-I could use close to 90mbps. Now this sounds like it would not work, even with a fast SD card (95mbps) because the camera can't write that fast through it's slower SD slot. This seems to be the thoughts of 5d cameramen I meet, but none of them actually have any experience with the SD not working. They've never tried.

Now, I have a 64GB 95mbps SD card (as well as a bunch of smaller CFs) and it all started when I was in a pinch shooting doc stuff a couple times and shot on the SD card. It seems like it worked perfectly fine! I recorded long clips too (close to the half hour limit) without it stopping or hitting some sort of buffer. I've looked at the footage, and edited some of it and there doesn't seem to be any issues. I've now shot on the SD card a good number of times (always at 1080p All-i 24fps). It has never stopped. It has never had corrupted files. It seems 100% ok.

The question I have is, am I missing something?! Is the camera doing some extra compression on my footage to keep up with the slow SD write speed? Am I losing color information or something? Am I somehow dropping frames and not noticing at all in post?

Basically, I need a bunch more media for documentary shoots. The large CF cards are so much more expensive than the SD cards. Is CF really necessary? Is it really more reliable? Is there anyone with a real world test that shows any limitations or is it just superstitious camera guys who just assume it doesn't work and therefore have never tried?

If the SD is really losing something, I'll happily keep it around for only photos and buy 64GB CFs. But if not, why not be SD as much as possible!

Yea, for the record, I have the San Disk Extreme Pro 64GB SD 95mbps card (and don't seem to be having any noticeable problems).

My problem is that I am repeatedly on shoots with other camera ops (hired by them) and they basically act like my SD card is not useable and require I shoot 100% on CF. If I had a definitive reason that they are wrong, I'd say so, but I want to know for sure.

Also, an interesting topic to ask about (maybe slightly off topic), is anyone has problems with larger GB cards? Is the only reason people stick to smaller ones a combination of paranoia of losing a whole day of shooting on 1 card and price? Or is there a legitimate worry of the Mark iii handling writing to large (64 GB or 128 GB cards)? I ask because for Documentary work, very large cards are definitely preferred.

I've not seen any questions about the SD cards reliability on the 5Dmkiii in recent posts, and I assume we would have if there was a problem. My own experience, on other non 5D HDSLR's was that the SD cards seemed less reliable. I did have card failures, but since moving to CF cards, both on 7Ds, 5Ds and xf100 & 300s, I've not experienced any card failures.. Maybe I'm just lucky. A sample of 1 person, even with 10 cards, is not a great sample. I do believe that the SD card connection is a physical slide in connection, and I always worry about touching that connection over and over, so I try not to. The CF cards connectors are hidden, so less likely to be corrupted by skin oil, or whatever else might be on your fingers if you accidentally touched the connectors.

I never experienced SD card failures on my HMC150, when I owned one, either.

I do worry about shooting 64GB cards, for the very reason you mention. The discipline of not putting 'all your eggs in 1 basket" is worth it for me, though to be clear, when I started buying 32GB cards, people were saying the same thing! Ultimately, if I'm worried about losing a shoot, I pickup the xf305, with dual redundant recording.

I use 16 & 32 gb CF cards with no issues. I am also of the belief that breaking up your shoot on different cards will allow more piece of mind in case of a card failure, thus limiting major loss of footage.

Not sure if you got to the bottom of your query on card speeds and why the Canon 5DIII can write video without problem to the SD card but I think the problem relates to the confusion between bits and bytes.

The 90Mbits figure that is quoted for the Canon 5DIII for ALL-I video is not the same as 90Mbytes per second - in fact 90Mbits converts to around just 11Mbytes per second which most reasonable SD cards can handle.

And this link to the write speed comparison done by Rob Galbraith - from the RAW and JPEG write speeds to file you can see all but the lowest can handle ALL-I video - not sure if anyone has tried one of those cards from the bottom of his list and whether the camera freezes or just removes the options for higher bit rate video?